A sailor threatened with prosecution in a 2009 rape case was found hanged, a reported suicide. But questions in the rape case have fueled suspicion that he was killed.

Family attorney James Culp, second from right, was on hand when Kyle Antonacci's…

The night began like many at Boorda Hall, a five-story barracks at Naval Station Great Lakes, the Navy's premier training base on the shore of Lake Michigan in Illinois.

Somebody announced a party, and the hard drinking and beer pong began. A 21-year-old Marine lance corporal, so drunk on rum and Mountain Dew she was slurring her words, went to look for Kyle Antonacci, a Navy seaman she'd been dating off and on. Antonacci soon texted his friend Mike Pineda to help him deal with her.

Both men had sex with her that night. But what distinguished May 8, 2009, from dozens of other party nights in the barracks — what turned it into a mystery that investigators still are trying to unravel — was what happened afterward.

Pineda, a seaman from Barstow, Calif., training as a Navy SEAL, spent three months in jail for sexual assault. Antonacci, a 22-year-old ordnance disposal trainee from Long Island, N.Y., was threatened with prosecution. The case didn't go far: Antonacci's body was found hanging in a closet, his nose bleeding, his face and back bruised.

The armed forces face a skyrocketing number of sexual assault cases. This investigation in particular — involving dozens of witnesses, lie detector tests and forensic exams — points up the difficulty of pursuing such prosecutions in the barracks, where drinking and sex often go hand in hand.

Witnesses, their memories clouded by alcohol, told stories that omitted key details. Antonacci admitted lying. The Navy told Antonacci's parents that he was a troubled young man who had taken his own life. But the parents never believed it, and neither did Pineda.

For years, the parents and Pineda, 31, have tried to prove that the Naval Criminal Investigative Service hid the truth to conceal its own bungled rape investigation. And they are not alone. The former Lake County coroner says he was denied access to key evidence and pressured to call the February 2010 death a suicide. He now believes Antonacci was murdered.

"They used him as a pawn in a game to win a case, and somebody needs to answer for that," Pineda said.

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Antonacci was popular in the barracks — breezy and funny in a New York-tough kind of way. The lance corporal who alleged the rape was the same: fun-loving and friendly. She and Antonacci had had casual sex on a few occasions; with Pineda, her acquaintance was platonic. She'd gone to his room a few times to watch TV and have dinner. (The woman is unnamed because The Times does not identify alleged victims in sexual assault cases.)

Antonacci said the woman had come on to him in his room that night. When Pineda arrived, Antonacci left. The woman told police she was so drunk that she lay down on the bed and blacked out, and the next thing she knew, Pineda was on top of her, having sex. Pineda said she was drunk, but fully awake. After they had sex and he got up to leave, he said, she called him back and demanded a kiss. "This didn't happen," Pineda said she told him.

Antonacci told authorities he returned and found the woman naked in his bed. She began to cry and told him she "didn't want to with Pineda." Later that night, she would angrily tell police she'd been sexually assaulted. Pineda was charged with "aggravated sexual assault by substantial incapacitation," meaning she had been too drunk to say yes.

Antonacci initially told authorities that he had left the woman lying on the bed, and heard Pineda lock the door as he left. But he told friends he later had come to believe the woman hadn't been unconscious at all. He said he thought she had lied about the assault because she was in love with him and didn't want him to think she had willingly had sex with his friend.

Furious at hearing this, the woman came to his room at 4:30 in the morning, warning him not to change his story, Antonacci said in a report to police. Minutes later, a Marine Corps friend of hers wielding a knife issued an even more belligerent warning, his report said. A large X was carved into the door of his room. Navy authorities moved Antonacci to another room when he requested protection.

"He was scared.... He literally didn't know what to do," said his sister, Karissa Phillip, who got frequent calls from her brother during those weeks.

At Pineda's court-martial in November 2009, Antonacci revealed neither his doubts about the woman's story nor the fact that he'd had sex with her that night, too. The jury found Pineda guilty, and Antonacci rushed up to his friend as he was being led to a military police van in shackles. He seemed stricken.

"I just looked at him and I said, 'It's not your fault, man,'" Pineda recalled. "And he fell to his knees and started crying."

After that, friends and family say, Antonacci was guilt-ridden. "He said, 'I know he didn't do it.' He knew this girl was lying, and he said he would do anything he could to help his friend get out of jail," Phillip recalled.